It is known that organic solvents such as chlorinated ethenes and other DNAPL substances can be broken down by exposure of the DNAPL substance to a strong oxidant.
It is also known to treat groundwater contaminated with chlorinated ethenes in-situ, i.e the contaminated groundwater is treated in the aquifer through which the groundwater is passing.
Conventionally, the treatment has been done by flushing, i.e by injecting the oxidant into the ground upstream of the contaminant, and drawing water out downstream, the intention being to pass the oxidant over and through the contaminated zone, and thereby bring about the destruction of the contaminant.
The oxidation reaction transforms the chlorine component of the contaminant into a harmless chloride salt. The resultant chloride, as produced by the oxidation reaction, may be ignored, at least in the small concentrations that are produced.
Oxidants that have been used conventionally to break down chlorinated ethenes include hydrogen peroxide, and permanganate, such as potassium permanganate. As mentioned, these substances have been injected upstream and drawn off downstream, the intention being to promote a flushing action. But conventional flushing, as a way of remediating DNAPL contaminants, is inefficient, and can be incomplete. The invention is aimed at promoting the breakdown reaction, for example the oxidation reaction, in a way that is substantially more efficient, and more economical, than flushing.
The invention is aimed at causing destruction of the contaminants in whatever form they occur in the ground, whether they are in the DNAPL form, sorbed on soil particles or dissolved in the groundwater. In the invention the treatment chemical in liquid form, that is substantially denser than water such as a permanganate solution for oxidation of the contaminants, is injected into the aquifer from a borehole so as to react with the contaminants. While the treatment liquid is being injected, it spreads out quickly into the aquifer to form a discrete zone extending from the borehole. The borehole used for the injection allows the treatment fluid to enter the aquifer only in a specified discrete open interval along the vertical extent of the borehole. For example, this open vertical interval may be 0.5 meters long. Thus, as the treatment liquid is injected through the open interval under gentle or moderate pressure, it forms a disc-like or ellipsoid-like zone substantially in the lateral direction in the aquifer. As the episode of applied injection pressure for the zone comes to an end, the treatment solution in the aquifer begins to spread laterally and also sink downward because of the effect of the density of the treatment solution and, as this spreading and sinking occurs, the treatment chemical also spreads because of molecular diffusion. Hence, these spreading processes cause the treatment chemical to invade a much larger volume of the aquifer than occurs during the injection period. To achieve the full advantage of this invasion, a period of time much longer than the injection period must pass to allow the spreading caused by density and diffusion to become complete or nearly so. As the treatment solution comes into contact with contaminants, it reacts with the contaminants to cause destruction of contaminant molecules. As this destruction occurs, treatment solution is consumed. The treatment chemical can also be consumed by reactions with the geological materials or other natural constituents in the aquifer. In some circumstances the mass of treatment solution put into the aquifer during the episode is not sufficient to destroy all of the contamination in the injected and invaded zones and therefore, another injection episode at or near this location may be needed. This second injection episode should occur after the invasion resulting from the first injection episode is complete or nearly so. Additional injection episodes may be needed to complete the desired degree of cleanup of the treatment zone. Thus, the full treatment of the targeted volume of aquifer may be achieved with only one injection episode or more episodes.
An aim of the invention is to provide versatility of treatment types, including its use to treat a specific layer or lens of contamination that has been found during investigations of the site. It also can be used to treat portions of an aquifer in which contaminant occurrence is known or suspected but the exact locations have not been determined. In the first type of use of the invention, the treatment may be focused towards a specific layer or lens-like zone of contamination. In contrast, in the second type of use, the invention may be used to enable the treatment chemicals eventually to invade the full volume of aquifer where treatment is desired but the whereabouts of the contaminants in this volume is not known in any detail.
Thus, the invention may be used to blanket a targeted volume of aquifer so that the treatment chemical will reach whatever contamination occurs in the volume. An important advantage of the invention is that the injection of the treatment liquid to form the immediate disc-like or ellipse-like zones in the aquifer is designed to occur in a manner that causes very little pushing away or displacement of the contaminated groundwater existing in the volume of aquifer being treated. After the injection period the spreading by density and diffusion causes no substantial displacement. This is much different from the flushing approach in which the treatment solution is forced by injection and, perhaps also by pumping of other wells nearby, to flow through the entire zone of the intended treatment zone. This is because the application of injection pressure is continual. It is particularly important, while blanketing the aquifer zone, to avoid pushing the contaminated groundwater out in front of the injected liquid because the water that is pushed out in front of the treatment liquid cannot then be treated. The systems as described herein avoid this excessive pushing-away of the contaminated water because when using the systems, two or more of the injected disc-like zones are formed in each borehole, one below the other, so that an initial gap or space exists between the disc-like zones. The gaps are substantially larger than the vertical heights of the injected discs after the injection period. The gaps are then filled in by the treatment chemical, because of the effects of density and diffusion. As this filling-in occurs, no lateral displacement of the contaminated groundwater takes place and therefore the contaminated groundwater in the gaps is treated, as well as the sorbed contaminants on the aquifer particles and any DNAPL that may occur in the gaps. The amount of treatment chemical injected during one episode may not be sufficient to accomplish the desired degree of treatment and therefore another injection episode later on at one or more of the depth levels in the borehole may be desired. Hence, the intended degree of treatment may be achieved by application of the treatment liquid episodically.
In addition to filling-in the vertical gaps between the initial disc-like zones, the treatment chemicals spread laterally beyond the outer limits of the initial disc because of the effect of the density of the treatment liquid. This lateral spreading is fostered by natural stratification that occurs in nearly all granular aquifers.
In aquifers where the DNAPL or other contaminants occur on top of or in the upper part of a lower permeability layer such as a silty or clayey layer the systems as described can be used efficiently to deliver the treatment chemicals to this zone. The injected disc-like zone of treatment liquid is formed by injection into the more permeable zone above but close to the silty or clayey layer. Then, the dense treatment liquid sinks onto and into the contamination and at the same time spreads out laterally along the top of the layer because of the density effect. Hence, the treatment liquid seeks out the contamination under its own influence. This spreading out effect enables a much larger area to be covered without having to continue the injection period. As the density effect causes the treatment liquid to spread out laterally along the top of the lower permeability layer, the vertical height (i.e. thickness) of the zone of treatment liquid generally becomes thinner so that outward displacement of contaminated water in front of the treatment zone is very small. Hence, the treatment chemical is caused to seek out the contaminant layer efficiently so that only a minimal amount of the treatment solution is consumed by reaction with the natural aquifer material. This leaves nearly all of the treatment chemical available to destroy the contaminants. If the mass of contamination in the zone along the silty or clayey layer is considerable, more than one injection episode may be desired. In this case, the treatment may be done episodically.
When the treatment chemical such as permanganate comes into contact with the contaminant, it reacts with it to cause destruction of contaminant molecules and, at the same time molecules of the treatment chemical are consumed by the reaction. This process of consumption of the treatment chemical causes the treatment liquid not to spread or sink as much as it would otherwise. This is another reason why a second or even additional injection episodes may be necessary at a single injection location so that the treatment liquid can be made eventually to invade or occupy the full volume or area for which treatment is desired.
The reaction of the treatment chemical with the contaminant causes consumption of molecules of the treatment chemical at the particular locations where the contaminants exist. This decline in concentration of the treatment chemical causes diffusion to continually move more treatment molecules towards the contamination, and contaminants towards the treatment solution. Thus, the injection and density-induced spreading of the treatment liquid need not bring the treatment molecules exactly into contact with the contaminants because diffusion also works to bring this contact into effect.